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To make an executable program, the GHC system compiles your code and
then links it with a non-trivial runtime system (RTS), which handles
storage management, thread scheduling, profiling, and so on.

The RTS has a lot of options to control its behaviour. For example, you
can change the context-switch interval, the default size of the heap,
and enable heap profiling. These options can be passed to the runtime
system in a variety of different ways; the next section
(Setting RTS options) describes the various methods, and the
following sections describe the RTS options themselves.

The RTS will snaffle -H32m-S for itself, and the remaining
arguments -f-hfoobar will be available to your program if/when it
calls System.Environment.getArgs.

No -RTS option is required if the runtime-system options extend to
the end of the command line, as in this example:

% hls -ltr /usr/etc +RTS -A5m

If you absolutely positively want all the rest of the options in a
command line to go to the program (and not the RTS), use a
--RTS.

As always, for RTS options that take ⟨size⟩s: If the last character of
⟨size⟩ is a K or k, multiply by 1000; if an M or m, by 1,000,000; if a G
or G, by 1,000,000,000. (And any wraparound in the counters is your
fault!)

Giving a +RTS-? RTS option option will print out the RTS
options actually available in your program (which vary, depending on how
you compiled).

Note

Since GHC is itself compiled by GHC, you can change RTS options in
the compiler using the normal +RTS...-RTS combination. For instance, to set
the maximum heap size for a compilation to 128M, you would add
+RTS-M128m-RTS to the command line.

GHC lets you change the default RTS options for a program at compile
time, using the -with-rtsopts flag (Options affecting linking). A common
use for this is to give your program a default heap and/or stack size
that is greater than the default. For example, to set -H128m-K64m,
link with -with-rtsopts="-H128m-K64m".

If the -rtsopts flag is set to something other than none or ignoreAll
when linking, RTS options are also taken from the environment variable
GHCRTS. For example, to set the maximum heap size to 2G
for all GHC-compiled programs (using an sh-like shell):

GHCRTS='-M2G'export GHCRTS

RTS options taken from the GHCRTS environment variable can be
overridden by options given on the command line.

Tip

Setting something like GHCRTS=-M2G in your environment is a
handy way to avoid Haskell programs growing beyond the real memory in
your machine, which is easy to do by accident and can cause the machine
to slow to a crawl until the OS decides to kill the process (and you
hope it kills the right one).

GHC lets you exercise rudimentary control over certain RTS settings for
any given program, by compiling in a “hook” that is called by the
run-time system. The RTS contains stub definitions for these hooks, but
by writing your own version and linking it on the GHC command line, you
can override the defaults.

Owing to the vagaries of DLL linking, these hooks don’t work under
Windows when the program is built dynamically.

If yes (the default), the RTS installs signal handlers to catch
things like Ctrl-C. This option is primarily useful for when you are
using the Haskell code as a DLL, and want to set your own signal
handlers.

Note that even with --install-signal-handlers=no, the RTS
interval timer signal is still enabled. The timer signal is either
SIGVTALRM or SIGALRM, depending on the RTS configuration and OS
capabilities. To disable the timer signal, use the -V0 RTS
option (see -V⟨secs⟩).

If yes (the default), the RTS on Windows installs exception handlers to
catch unhandled exceptions using the Windows exception handling mechanism.
This option is primarily useful for when you are using the Haskell code as a
DLL, and don’t want the RTS to ungracefully terminate your application on
erros such as segfaults.

If yes (the default), the RTS on Windows will generate a core dump on
any crash. These dumps can be inspected using debuggers such as WinDBG.
The dumps record all code, registers and threading information at the time
of the crash. Note that this implies –install-seh-handlers=yes.

If yes (the default), the RTS on Windows will generate a stack trace on
crashes if exception handling are enabled. In order to get more information
in compiled executables, C code or DLLs symbols need to be available.

This option is for working around memory allocation
problems only. Do not use unless GHCi fails with a message like
“failedtommap()memorybelow2Gb”. If you need to use this
option to get GHCi working on your machine, please file a bug.

On 64-bit machines, the RTS needs to allocate memory in the low 2Gb
of the address space. Support for this across different operating
systems is patchy, and sometimes fails. This option is there to give
the RTS a hint about where it should be able to allocate memory in
the low 2Gb of the address space. For example,
+RTS-xm20000000-RTS would hint that the RTS should allocate
starting at the 0.5Gb mark. The default is to use the OS’s built-in
support for allocating memory in the low 2Gb if available (e.g.
mmap with MAP_32BIT on Linux), or otherwise -xm40000000.

This option relates to allocation limits; for more about this see
GHC.Conc.enableAllocationLimit.
When a thread hits its allocation limit, the RTS throws an exception
to the thread, and the thread gets an additional quota of allocation
before the exception is raised again, the idea being so that the
thread can execute its exception handlers. The -xq controls the
size of this additional quota.

There are several options to give you precise control over garbage
collection. Hopefully, you won’t need any of these in normal operation,
but there are several things that can be tweaked for maximum
performance.

Set the allocation area size used by the garbage
collector. The allocation area (actually generation 0 step 0) is
fixed and is never resized (unless you use -H[⟨size⟩], below).

Increasing the allocation area size may or may not give better
performance (a bigger allocation area means worse cache behaviour
but fewer garbage collections and less promotion).

With only 1 generation (e.g. -G1, see -G⟨generations⟩) the
-A option specifies the minimum allocation area, since the actual size
of the allocation area will be resized according to the amount of data in
the heap (see -F⟨factor⟩, below).

Sets the limit on the total size of “large objects” (objects
larger than about 3KB) that can be allocated before a GC is
triggered. By default this limit is the same as the -A value.

Large objects are not allocated from the normal allocation area
set by the -A flag, which is why there is a separate limit for
these. Large objects tend to be much rarer than small objects, so
most programs hit the -A limit before the -AL limit. However,
the -A limit is per-capability, whereas the -AL limit is global,
so as -N gets larger it becomes more likely that we hit the
-AL limit first. To counteract this, it might be necessary to
use a larger -AL limit when using a large -N.

To see whether you’re making good use of all the memory reseverd
for the allocation area (-A times -N), look at the output of
+RTS-S and check whether the amount of memory allocated between
GCs is equal to -A times -N. If not, there are two possible
remedies: use -n to set a nursery chunk size, or use -AL to
increase the limit for large objects.

Set the minimum size of the old generation. The old generation is collected
whenever it grows to this size or the value of the -F⟨factor⟩
option multiplied by the size of the live data at the previous major
collection, whichever is larger.

[Example: -n4m ] When set to a non-zero value, this
option divides the allocation area (-A value) into chunks of the
specified size. During execution, when a processor exhausts its
current chunk, it is given another chunk from the pool until the
pool is exhausted, at which point a collection is triggered.

This option is only useful when running in parallel (-N2 or
greater). It allows the processor cores to make better use of the
available allocation area, even when cores are allocating at
different rates. Without -n, each core gets a fixed-size
allocation area specified by the -A, and the first core to
exhaust its allocation area triggers a GC across all the cores. This
can result in a collection happening when the allocation areas of
some cores are only partially full, so the purpose of the -n is
to allow cores that are allocating faster to get more of the
allocation area. This means less frequent GC, leading a lower GC
overhead for the same heap size.

This is particularly useful in conjunction with larger -A
values, for example -A64m-n4m is a useful combination on larger core
counts (8+).

Use a compacting algorithm for collecting the oldest generation. By
default, the oldest generation is collected using a copying
algorithm; this option causes it to be compacted in-place instead.
The compaction algorithm is slower than the copying algorithm, but
the savings in memory use can be considerable.

For a given heap size (using the -H⟨size⟩ option), compaction
can in fact reduce the GC cost by allowing fewer GCs to be performed. This
is more likely when the ratio of live data to heap size is high, say
greater than 30%.

Note

Compaction doesn’t currently work when a single generation is
requested using the -G1 option.

Automatically enable compacting collection when the live data exceeds ⟨n⟩%
of the maximum heap size (see the -M⟨size⟩ option). Note that
the maximum heap size is unlimited by default, so this option has no effect
unless the maximum heap size is set with -M⟨size⟩.

This option controls the amount of memory reserved for
the older generations (and in the case of a two space collector the
size of the allocation area) as a factor of the amount of live data.
For example, if there was 2M of live data in the oldest generation
when we last collected it, then by default we’ll wait until it grows
to 4M before collecting it again.

The default seems to work well here. If you have plenty of memory, it is
usually better to use -H⟨size⟩ (see -H[⟨size⟩]) than to
increase -F⟨factor⟩.

The -F⟨factor⟩ setting will be automatically reduced by the garbage
collector when the maximum heap size (the -M⟨size⟩ setting) is approaching.

Set the number of generations used by the garbage
collector. The default of 2 seems to be good, but the garbage
collector can support any number of generations. Anything larger
than about 4 is probably not a good idea unless your program runs
for a long time, because the oldest generation will hardly ever
get collected.

Specifying 1 generation with +RTS-G1 gives you a simple 2-space
collector, as you would expect. In a 2-space collector, the -A⟨size⟩ option specifies the minimum allocation area size, since the
allocation area will grow with the amount of live data in the heap. In a
multi-generational collector the allocation area is a fixed size (unless
you use the -H[⟨size⟩] option).

Use parallel GC in generation ⟨gen⟩ and higher. Omitting ⟨gen⟩ turns off the
parallel GC completely, reverting to sequential GC.

The default parallel GC settings are usually suitable for parallel programs
(i.e. those using GHC.Conc.par, Strategies, or with
multiple threads). However, it is sometimes beneficial to enable the
parallel GC for a single-threaded sequential program too, especially if the
program has a large amount of heap data and GC is a significant fraction of
runtime. To use the parallel GC in a sequential program, enable the parallel
runtime with a suitable -N⟨x⟩ option, and additionally it might
be beneficial to restrict parallel GC to the old generation with -qg1.

Use load-balancing in the parallel GC in generation ⟨gen⟩ and higher.
Omitting ⟨gen⟩ disables load-balancing entirely.

Load-balancing shares out the work of GC between the available
cores. This is a good idea when the heap is large and we need to
parallelise the GC work, however it is also pessimal for the short
young-generation collections in a parallel program, because it can
harm locality by moving data from the cache of the CPU where is it
being used to the cache of another CPU. Hence the default is to do
load-balancing only in the old-generation. In fact, for a parallel
program it is sometimes beneficial to disable load-balancing
entirely with -qb.

By default, all of the capabilities participate in parallel
garbage collection. If we want to use a very large -N value,
however, this can reduce the performance of the GC. For this
reason, the -qn flag can be used to specify a lower number for
the threads that should participate in GC. During GC, if there
are more than this number of workers active, some of them will
sleep for the duration of the GC.

The -qn flag may be useful when running with a large -A value
(so that GC is infrequent), and a large -N value (so as to make
use of hyperthreaded cores, for example). For example, on a
24-core machine with 2 hyperthreads per core, we might use
-N48-qn24-A128m to specify that the mutator should use
hyperthreads but the GC should only use real cores. Note that
this configuration would use 6GB for the allocation area.

This option provides a “suggested heap size” for the garbage collector.
Think of -Hsize as a variable -A⟨size⟩ option. It says: I
want to use at least ⟨size⟩ bytes, so use whatever is left over to increase
the -A value.

This option does not put a limit on the heap size: the heap may
grow beyond the given size as usual.

If ⟨size⟩ is omitted, then the garbage collector will take the size
of the heap at the previous GC as the ⟨size⟩. This has the effect of
allowing for a larger -A value but without increasing the
overall memory requirements of the program. It can be useful when
the default small -A value is suboptimal, as it can be in
programs that create large amounts of long-lived data.

In the threaded and SMP versions of the RTS (see
-threaded, Options affecting linking), a major GC is automatically
performed if the runtime has been idle (no Haskell computation has
been running) for a period of time. The amount of idle time which
must pass before a GC is performed is set by the -I⟨seconds⟩
option. Specifying -I0 disables the idle GC.

For an interactive application, it is probably a good idea to use
the idle GC, because this will allow finalizers to run and
deadlocked threads to be detected in the idle time when no Haskell
computation is happening. Also, it will mean that a GC is less
likely to happen when the application is busy, and so responsiveness
may be improved. However, if the amount of live data in the heap is
particularly large, then the idle GC can cause a significant delay,
and too small an interval could adversely affect interactive
responsiveness.

This is an experimental feature, please let us know if it causes
problems and/or could benefit from further tuning.

Thread stacks (including the main thread’s stack) live on the heap.
As the stack grows, new stack chunks are added as required; if the
stack shrinks again, these extra stack chunks are reclaimed by the
garbage collector. The default initial stack size is deliberately
small, in order to keep the time and space overhead for thread
creation to a minimum, and to make it practical to spawn threads for
even tiny pieces of work.

Note

This flag used to be simply -k, but was renamed to -ki in
GHC 7.2.1. The old name is still accepted for backwards
compatibility, but that may be removed in a future version.

Set the size of “stack chunks”. When a thread’s current stack overflows, a
new stack chunk is created and added to the thread’s stack, until the limit
set by -K⟨size⟩ is reached.

The advantage of smaller stack chunks is that the garbage collector can
avoid traversing stack chunks if they are known to be unmodified since the
last collection, so reducing the chunk size means that the garbage
collector can identify more stack as unmodified, and the GC overhead might
be reduced. On the other hand, making stack chunks too small adds some
overhead as there will be more overflow/underflow between chunks. The
default setting of 32k appears to be a reasonable compromise in most cases.

Sets the stack chunk buffer size. When a stack chunk
overflows and a new stack chunk is created, some of the data from
the previous stack chunk is moved into the new chunk, to avoid an
immediate underflow and repeated overflow/underflow at the boundary.
The amount of stack moved is set by the -kb option.

Note that to avoid wasting space, this value should typically be less than
10% of the size of a stack chunk (-kc⟨size⟩), because in a
chain of stack chunks, each chunk will have a gap of unused space of this
size.

Set the maximum stack size for
an individual thread to ⟨size⟩ bytes. If the thread attempts to
exceed this limit, it will be sent the StackOverflow exception.
The limit can be disabled entirely by specifying a size of zero.

This option is there mainly to stop the program eating up all the
available memory in the machine if it gets into an infinite loop.

Set the maximum heap size to ⟨size⟩ bytes. The
heap normally grows and shrinks according to the memory requirements
of the program. The only reason for having this option is to stop
the heap growing without bound and filling up all the available swap
space, which at the least will result in the program being summarily
killed by the operating system.

The maximum heap size also affects other garbage collection
parameters: when the amount of live data in the heap exceeds a
certain fraction of the maximum heap size, compacting collection
will be automatically enabled for the oldest generation, and the
-F parameter will be reduced in order to avoid exceeding the
maximum heap size.

If the program’s heap exceeds the value set by -M⟨size⟩, the
RTS throws an exception to the program, and the program gets an
additional quota of allocation before the exception is raised
again, the idea being so that the program can execute its
exception handlers. -Mgrace= controls the size of this
additional quota.

Enable NUMA-aware memory allocation in the runtime (only available
with -threaded, and only on Linux currently).

Background: some systems have a Non-Uniform Memory Architecture,
whereby main memory is split into banks which are “local” to
specific CPU cores. Accessing local memory is faster than
accessing remote memory. The OS provides APIs for allocating
local memory and binding threads to particular CPU cores, so that
we can ensure certain memory accesses are using local memory.

The --numa option tells the RTS to tune its memory usage to
maximize local memory accesses. In particular, the RTS will:

Perform other memory allocation, including in the GC, from
node-local memory.

When load-balancing, we prefer to migrate threads to another
Capability on the same node.

The --numa flag is typically beneficial when a program is
using all cores of a large multi-core NUMA system, with a large
allocation area (-A). All memory accesses to the allocation
area will go to local memory, which can save a significant amount
of remote memory access. A runtime speedup on the order of 10%
is typical, but can vary a lot depending on the hardware and the
memory behaviour of the program.

Note that the RTS will not set CPU affinity for bound threads and
threads entering Haskell from C/C++, so if your program uses bound
threads you should ensure that each bound thread calls the RTS API
rts_setInCallCapability(c,1) from C/C++ before calling into
Haskell. Otherwise there could be a mismatch between the CPU that
the thread is running on and the memory it is using while running
Haskell code, which will negate any benefits of --numa.

If given an explicit <mask>, the <mask> is interpreted as a bitmap
that indicates the NUMA nodes on which to run the program. For
example, --numa=3 would run the program on NUMA nodes 0 and 1.

When a GC starts, all the running mutator threads have to stop and
synchronise. The period between when the GC is initiated and all
the mutator threads are stopped is called the GC synchronisation
phase. If this phase is taking a long time (longer than 1ms is
considered long), then it can have a severe impact on overall
throughput.

A long GC sync can be caused by a mutator thread that is inside an
unsafe FFI call, or running in a loop that doesn’t allocate
memory and so doesn’t yield. To fix the former, make the call
safe, and to fix the latter, either avoid calling the code in
question or compile it with -fomit-yields.

By default, the flag will cause a warning to be emitted to stderr
when the sync time exceeds the specified time. This behaviour can
be overriden, however: the longGCSync() hook is called when
the sync time is exceeded during the sync period, and the
longGCSyncEnd() hook at the end. Both of these hooks can be
overriden in the RtsConfig when the runtime is started with
hs_init_ghc(). The default implementations of these hooks
(LongGcSync() and LongGCSyncEnd() respectively) print
warnings to stderr.

One way to use this flag is to set a breakpoint on
LongGCSync() in the debugger, and find the thread that is
delaying the sync. You probably want to use -g to
provide more info to the debugger.

The GC sync time, along with other GC stats, are available by
calling the getRTSStats() function from C, or
GHC.Stats.getRTSStats from Haskell.

These options produce runtime-system statistics, such as the amount
of time spent executing the program and in the garbage collector,
the amount of memory allocated, the maximum size of the heap, and so
on. The three variants give different levels of detail: -T
collects the data but produces no output -t produces a single
line of output in the same format as GHC’s -Rghc-timing option,
-s produces a more detailed summary at the end of the program,
and -S additionally produces information about each and every
garbage collection.

The output is placed in ⟨file⟩. If ⟨file⟩ is omitted, then the
output is sent to stderr.

If you use the -T flag then, you should access the statistics
using GHC.Stats.

If you use the -t flag then, when your program finishes, you
will see something like this:

The total number of bytes allocated by the program over the whole
run.

The total number of garbage collections performed.

The average and maximum “residency”, which is the amount of live
data in bytes. The runtime can only determine the amount of live
data during a major GC, which is why the number of samples
corresponds to the number of major GCs (and is usually relatively
small). To get a better picture of the heap profile of your
program, use the -hT RTS option (RTS options for profiling).

The peak memory the RTS has allocated from the OS.

The amount of CPU time and elapsed wall clock time while
initialising the runtime system (INIT), running the program
itself (MUT, the mutator), and garbage collecting (GC).

You can also get this in a more future-proof, machine readable
format, with -t--machine-readable:

If you use the -s flag then, when your program finishes, you
will see something like this (the exact details will vary depending
on what sort of RTS you have, e.g. you will only see profiling data
if your RTS is compiled for profiling):

The “bytes allocated in the heap” is the total bytes allocated by
the program over the whole run.

GHC uses a copying garbage collector by default. “bytes copied
during GC” tells you how many bytes it had to copy during garbage
collection.

The maximum space actually used by your program is the “bytes
maximum residency” figure. This is only checked during major
garbage collections, so it is only an approximation; the number
of samples tells you how many times it is checked.

The “bytes maximum slop” tells you the most space that is ever
wasted due to the way GHC allocates memory in blocks. Slop is
memory at the end of a block that was wasted. There’s no way to
control this; we just like to see how much memory is being lost
this way.

The “total memory in use” tells you the peak memory the RTS has
allocated from the OS.

Next there is information about the garbage collections done. For
each generation it says how many garbage collections were done,
how many of those collections were done in parallel, the total
CPU time used for garbage collecting that generation, and the
total wall clock time elapsed while garbage collecting that
generation.

The SPARKS statistic refers to the use of
Control.Parallel.par and related functionality in the
program. Each spark represents a call to par; a spark is
“converted” when it is executed in parallel; and a spark is
“pruned” when it is found to be already evaluated and is
discarded from the pool by the garbage collector. Any remaining
sparks are discarded at the end of execution, so “converted” plus
“pruned” does not necessarily add up to the total.

Next there is the CPU time and wall clock time elapsed broken
down by what the runtime system was doing at the time. INIT is
the runtime system initialisation. MUT is the mutator time, i.e.
the time spent actually running your code. GC is the time spent
doing garbage collection. RP is the time spent doing retainer
profiling. PROF is the time spent doing other profiling. EXIT is
the runtime system shutdown time. And finally, Total is, of
course, the total.

%GC time tells you what percentage GC is of Total. “Alloc rate”
tells you the “bytes allocated in the heap” divided by the MUT
CPU time. “Productivity” tells you what percentage of the Total
CPU and wall clock elapsed times are spent in the mutator (MUT).

The -S flag, as well as giving the same output as the -s
flag, prints information about each GC as it happens:

Generates a basic heap profile, in the file prog.hp. To produce the
heap profile graph, use hp2ps (see hp2ps – Rendering heap profiles to PostScript). The basic heap
profile is broken down by data constructor, with other types of closures
(functions, thunks, etc.) grouped into broad categories (e.g. FUN,
THUNK). To get a more detailed profile, use the full profiling support
(Profiling). Can be shortened to -h.

Log events in binary format. Without any ⟨flags⟩ specified, this
logs a default set of events, suitable for use with tools like ThreadScope.

Per default the events are written to program.eventlog though
the mechanism for writing event log data can be overriden with a custom
EventLogWriter.

For some special use cases you may want more control over which
events are included. The ⟨flags⟩ is a sequence of zero or more
characters indicating which classes of events to log. Currently
these the classes of events that can be enabled/disabled:

For spark events there are two modes: sampled and fully accurate.
There are various events in the life cycle of each spark, usually
just creating and running, but there are some more exceptional
possibilities. In the sampled mode the number of occurrences of each
kind of spark event is sampled at frequent intervals. In the fully
accurate mode every spark event is logged individually. The latter
has a higher runtime overhead and is not enabled by default.

The format of the log file is described by the header
EventLogFormat.h that comes with GHC, and it can be parsed in
Haskell using the
ghc-events
library. To dump the contents of a .eventlog file as text, use
the tool ghc-eventsshow that comes with the
ghc-events
package.

Log events as text to standard output, instead of to the
.eventlog file. The ⟨flags⟩ are the same as for -l, with the
additional option t which indicates that the each event printed
should be preceded by a timestamp value (in the binary .eventlog
file, all events are automatically associated with a timestamp).

The debugging options -Dx also generate events which are logged
using the tracing framework. By default those events are dumped as text
to stdout (-Dx implies -v), but they may instead be stored in
the binary eventlog file by using the -l option.

Oddly enough, people really do use this option! Our pal in Durham
(England), Paul Callaghan, writes: “Some people here use it for a
variety of purposes—honestly!—e.g., confirmation that the
code/machine is doing something, infinite loop detection, gauging
cost of recently added code. Certain people can even tell what stage
[the program] is in by the beep pattern. But the major use is for
annoying others in the same office…”

An RTS debugging flag; only available if the program was linked with
the -debug option. Various values of ⟨x⟩ are provided to enable
debug messages and additional runtime sanity checks in different
subsystems in the RTS, for example +RTS-Ds-RTS enables debug
messages from the scheduler. Use +RTS-? to find out which debug
flags are supported.

Debug messages will be sent to the binary event log file instead of
stdout if the -l option is added. This might be useful for
reducing the overhead of debug tracing.

(Only available when the program is compiled for profiling.) When an
exception is raised in the program, this option causes a stack trace
to be dumped to stderr.

This can be particularly useful for debugging: if your program is
complaining about a head[] error and you haven’t got a clue
which bit of code is causing it, compiling with
-prof-fprof-auto (see -prof) and running with +RTS-xc-RTS will tell you exactly the call stack at the point the error was
raised.

The output contains one report for each exception raised in the
program (the program might raise and catch several exceptions during
its execution), where each report looks something like this:

*** Exception raised (reporting due to +RTS -xc), stack trace:
GHC.List.CAF
--> evaluated by: Main.polynomial.table_search,
called from Main.polynomial.theta_index,
called from Main.polynomial,
called from Main.zonal_pressure,
called from Main.make_pressure.p,
called from Main.make_pressure,
called from Main.compute_initial_state.p,
called from Main.compute_initial_state,
called from Main.CAF
...

The stack trace may often begin with something uninformative like
GHC.List.CAF; this is an artifact of GHC’s optimiser, which
lifts out exceptions to the top-level where the profiling system
assigns them to the cost centre “CAF”. However, +RTS-xc doesn’t
just print the current stack, it looks deeper and reports the stack
at the time the CAF was evaluated, and it may report further stacks
until a non-CAF stack is found. In the example above, the next stack
(after -->evaluatedby) contains plenty of information about
what the program was doing when it evaluated head[].

Implementation details aside, the function names in the stack should
hopefully give you enough clues to track down the bug.

See also the function traceStack in the module Debug.Trace
for another way to view call stacks.

The information is formatted such that it can be read as a of type
[(String,String)]. Currently the following fields are present:

GHCRTS

Is this program linked against the GHC RTS? (always “YES”).

GHCversion

The version of GHC used to compile this program.

RTSway

The variant (“way”) of the runtime. The most common values are
rts_v (vanilla), rts_thr (threaded runtime, i.e. linked
using the -threaded option) and rts_p (profiling runtime,
i.e. linked using the -prof option). Other variants include
debug (linked using -debug), and dyn (the RTS is linked
in dynamically, i.e. a shared library, rather than statically linked
into the executable itself). These can be combined, e.g. you might
have rts_thr_debug_p.

TargetplatformTargetarchitectureTargetOSTargetvendor

These are the platform the program is compiled to run on.

BuildplatformBuildarchitectureBuildOSBuildvendor

These are the platform where the program was built on. (That is, the
target platform of GHC itself.) Ordinarily this is identical to the
target platform. (It could potentially be different if
cross-compiling.)

HostplatformHostarchitectureHostOSHostvendor

These are the platform where GHC itself was compiled. Again, this
would normally be identical to the build and target platforms.

Wordsize

Either "32" or "64", reflecting the word size of the target
platform.

Compilerunregistered

Was this program compiled with an “unregistered”
version of GHC? (I.e., a version of GHC that has no
platform-specific optimisations compiled in, usually because this is
a currently unsupported platform.) This value will usually be no,
unless you’re using an experimental build of GHC.

Tablesnexttocode

Putting info tables directly next to entry code is a useful
performance optimisation that is not available on all platforms.
This field tells you whether the program has been compiled with this
optimisation. (Usually yes, except on unusual platforms.)